Operational amplifiers are widely used in the electronics industry because of their many excellent circuit characteristics including high open loop gain, high input impedance, and low output impedance. Operational transconductance amplifiers are similar to the operational amplifiers generally, but exhibit high output impedance. General applications of operational amplifiers include circuit configurations such as voltage and current amplifiers, differentiators and integrators, active filters, oscillators, and analog-to-digital and digital-to-analog converters. To realize these different circuit configurations, operational amplifiers are used in conjunction with positive and/or negative feedback combined with passive and/or active elements.
An operational amplifier is also widely used to function as a voltage comparator, wherein typically, a reference signal is applied to the inverting input, and the voltage to be compared is applied to the non-inverting input. If the magnitude of the voltage to be compared is greater than the magnitude of the reference signal, the output of the comparator equals the positive supply voltage. If the magnitude of the voltage to be compared is less than the magnitude of the reference voltage, the output of the comparator equals the negative or ground supply voltage. An inverted voltage comparator may be provided by simply transposing the signals at the inverting and non-inverting inputs. Using the operational amplifier as a voltage comparator requires no external components or feedback, and its output only has two states: high and low.
The operational amplifier, as utilized in the realization of a variety of circuit functions, may be manufactured in bipolar or Complementary Metal Oxide Semiconductor (CMOS) technology or some combination thereof. The CMOS implementation is desirable for its low power consumption characteristic. Also, operational amplifiers are increasingly being integrated onto chips which merge digital and analog functions together with an increasing number of devices.
Current operational amplifiers require bias voltages that are not equal to the available supply voltages. Hence, the bias voltages must be provided, either at an input pin to the operational amplifier, or in the case of an integrated operational amplifier, additional circuitry must be incorporated therein to provide the proper bias voltage magnitudes. Using an external input presents problems similar to those for the null offset, and the internal circuitry is subject to statistical process variations across the integrated circuit.
Still further, should the magnitude of the differential input voltages drop below a threshold of the bias voltages, then an undesirably high common mode current, internal to the operational amplifier circuitry, might flow, depending upon the given circuit application. Such common mode current presents a drain of battery power, which for many applications, such as in radio pagers, is to be avoided since battery life is a major design concern. One method of ensuring the common mode current does not drain the battery needlessly is to provide additional safeguard to ensure that the input voltages never fall out of a given range. This is not always feasible or economical.
Thus, what is needed is an operational amplifier that does not require additional circuitry for each bias voltage and has a mechanism for ensuring that the common mode current does not unnecessarily drain power when an input voltage magnitude falls below a bias voltage magnitude.